Cluster Server and Trusted Server

I have code in the Library that uses Maildb.Open.

When this Mail db is put on more that 1 server in a cluster, the agent calling the subroutine works fine as long as the server is the same as the users MailServer.

The agent fails if the db is on a cluster member.

I have the cluster members entered in the Trusted Server Field in the Mail Server Document

But the agent fails with error

4269 - Can’t open foreign mail server [ServerName] from this server

What could I be doing wrong?